1. Field of the Invention
The present invention relates to an apparatus and a method for applying a coating liquid to a cylindrical substrate and to an electrophotographic photoreceptor produced by that method and an electrophotographic apparatus provided with the same.
2. Description of the Related Art
Technologies for applying a coating liquid to a cylindrical substrate have hitherto been employed in various fields. Here, the preparation of an electrophotographic photoreceptor will be hereunder enumerated. Incidentally, even in limiting to the electrographic field, application of a coating liquid to a cylindrical substrate is employed in the preparation of not only electrophotographic photoreceptors but also charging rolls, transfer rolls, fixing rolls, and so on.
The electrophotographic photoreceptor includes ones in which a lamination type photosensitive layer is formed by successively applying a coating material for undercoating layer, a coating material for charge-generating layer and a coating material for charge-transporting layer as coating materials for electrophotographic photoreceptor to the peripheral surface of a hollow cylindrical substrate constituted of aluminum or the like. The photosensitive layer is not only required to have a thin and uniform thickness but also eagerly demanded to realize low costs. Accordingly, a coating method having excellent producibility is being developed and investigated.
As a method of forming a photosensitive layer by applying coating materials for electrophotographic photoreceptor to the peripheral surface of a cylindrical substrate, a spray coating method, a dip coating method, a blade coating method, and so on have hitherto been known. However, these related art coating methods involve such problems that a uniform coating film is not obtained and that the production efficiency is poor.
For example, the spray coating method involves such a problem that when a solvent having a low boiling point is used in the coating material for electrophotographic photoreceptor, the solvent contained in the coating material is vaporized on the way of arrival of the coating material at the peripheral surface of the substrate, whereby the concentration of solids in the coating material increases, and therefore, when the coating material arrives at the substrate, it does not sufficiently spread on the peripheral surface of the substrate, and the surface of the coating film becomes irregular so that a smooth coating film surface is not obtained, whereby a coating film having a uniform thickness is not obtained.
Conversely, when a solvent having a high boiling point is used, though after attachment of the coating material onto the peripheral surface of the substrate, an action for leveling the thickness (hereinafter referred to as “leveling”) is revealed, since vaporization of the solvent is slow, fixing of the coating film is delayed. When coating is continued in such a state that the fixing of the coating film is insufficient, in the case where a desired thickness is thick, there is encountered a problem that sagging of the coating material occurs so that a coating film having a uniform thickness is not obtained, either. In order to avoid this problem, the coating material may be dividedly coated several times. However, this method involves such problems that since coating and drying must be repeatedly carried out until the coating film becomes dry to the touch (the state that the coating film becomes dry to a degree that a trace does not remain even by finger touch), a time required for achieving works is long and that steps thereof are extremely complicated.
According to the dip coating method, though smoothness of the coating film surface is improved, the coating film is formed even in the interior and end face of the substrate. The coating film formed in the interior and end face of the substrate becomes an obstacle in installing a flange, etc. in the substrate. Accordingly, in order to formulate a substrate in which a coating film is formed in the interior and end face thereof into a substrate for electrophotographic photoreceptor, there is encountered a problem that the coating film formed in the interior and end face of the substrate must be peeled away. Also, in order to peel away the coating film formed in the interior and end face of the substrate, since a peeling step is necessary, such became a factor for hindering producibility. Further, since the thickness of the coating film is largely affected by physical properties of the coating material and a lifting rate after dipping, when lifting is carried out at a constant rate, a thickness difference between the upper end and the lower end of the substrate is generated. In order to overcome such a thickness difference, it is necessary to control the lifting rate. However, that control is difficult. Also, there is encountered a problem that in order to form a coating film having a uniform thickness, the lifting rate after dipping must be made slow. Thus, high production efficiency was not obtained.
The blade coating method is a coating method in which a blade is aligned in a position closed to the length direction of a substrate, and after making the substrate one revolution, the blade is moved backward. According to the blade coating method, though high producibility is obtained, there is encountered a problem that when the blade is moved backward, a phenomenon wherein a part of the coating film coated on the substrate swells occurs due to a surface tension of the substrate so that the thickness becomes non-uniform by this swelling.
Also, there is a roll coating method other than the foregoing methods. The roll coating method involves a problem caused by special characteristics of a substrate as a material to be coated that it is cylindrical, namely, residence of a coating material generated when a cylindrical substrate as a material to be coated rotates and the once coated surface returns repeatedly to a coating portion makes the thickness non-uniform.
As the related art technology of avoiding this residence of coating material from occurring, there is a method in which at the point of time when a substrate makes one revolution and the entire peripheral surface thereof is coated with a coating material, the substrate is kept away from a roll (see Japanese unexamined Patent Publication JP-A 3-12261 (1991)). However, according ( to the related art technology disclosed in JP-A 3-12261, in the case where the substrate makes only one revolution, there is encountered a problem that not only it is difficult to obtain a uniform coating film, but also a seam of the coating material generated when the substrate is kept away from the roll remains. Further, JP-A 3-12261 discloses a method in which after completion of coating by making the substrate one or more revolutions, the substrate is kept away from the coating material supplying roll, and rotation of the substrate is continued to level the coating film surface. However, this method involves such a problem that the thickness must be precisely controlled in expectation of an amount of the residence of the coating material to be subjected to leveling in advance and the substrate must be kept while rotating for a period of time necessary for achieving leveling, resulting in a lowering of the production efficiency.
Also, though a gravure offset method that has hitherto been employed is excellent as a method of forming a certain specific pattern with good precision, it is concerned with a technology fundamentally different from so-called “coating” of forming a uniform coating film and involves such a problem that when it is intended to form a coating film on a cylindrical substrate, a pattern of a plate remains, or a seam is formed.